Alicia Keys has promised to deliver "pimped-out vocals" from Christina Aguilera on "Impossible," a track she co-produced for Aguilera's upcoming album.

On day three of their sessions at New York's Electric Lady studios last month, Aguilera wasn't looking like the "big ol' pimp" Keys was boasting her to be. There was no diamond-encrusted gold goblet or neon three-piece suit with the matching hat and 'gators. X-tina just sported a cutoff Terry Bradshaw Pittsburgh Steelers jersey, though her white undershirt did match her pink and white scarf (Click for photos from the studio).

Aguilera's nose was stuffy, her eyes were puffy and she was coughing. Obviously her cold didn't have her much into a game-spitting mood  not that she would've been wasting time shooting the breeze if she were the perfect picture of health. Aguilera was ready to get down to business and finish the track, which was being produced by the Crucial Keys team of Alicia and her partner, Kerry Brothers.

"I like the piano," a giddy Aguilera said of the slow, chord-driven track Keys devised to complement her big voice. "Now I gotta learn how to play. You inspire [me]."

Looking low-key in sunshades, a scarf and a leather jacket, Keys returned the compliment. "I like them vocals," she said. "We're smashing. Official killer. 'Killer Chris!' "

A few minutes later, Keys, who was orchestrating the session when she wasn't laying background vocals or harmonizing with her collaborator, sat intently at the boards listening to K.C. finish the second verse.

" 'It's impossible to make it easy if you're always trying to make it so damn haaarrrd,' " the 22-year-old sang. " 'How can I, how can I give you all my love, baby/ If you're always, always putting up your guaaarrrd/ This is not a circus, don'tcha play me for a clown.' "

Keys and Aguilera have been wanting to work together for a while, Keys said. Keys wrote the new cut, but not before the Grammy Award-winners had a chance to vibe off each other  they hung out and partied before Keys penned the track.

A tentative September 17 release date has been set for Aguilera's still-untitled album, which is the proper follow-up to her 1999 self-titled debut.